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The Periodic Table

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Reading the Periodic Table

In 1789, building upon the work of precursors and contemporaries alike, the French
chemist Antoine Laurent Lavoisier first defined an element as a fundamental substance
that could not be broken down by any chemical means then known. In the same
Treatise on Chemical Elements, he compiled a list of 33 elements (a number of
which were not actually elements) and devised a naming system for the discovery of new
elements.

Lavoisier's definition and list of elements helped spur an attempt by chemists to systematize and
understand the elements. In 1803, the English chemist John Dalton used the general scientific
recognition that elements combined with each other according to different ratios by weight to create an
atomic theory that claimed all elements were built out of variable numbers of hydrogen atoms. As a part
of this theory, Dalton created a scale of atomic weight based on the hydrogen
atom (the weight of
hydrogen was set equal to 1). In 1869, the Russian chemist Dmitry Mendeleyev organized the elements in
a table according to their atomic weights (the German chemist Julius Lothar Meyer independently struck
upon the same organization in 1870).

In the sixty-seven years from Dalton's formulation of atomic
weight to Mendeleyev's periodic table many scientists had tried to create a working organizational
structure for the elements. Mendeleyev succeeded where others failed because he realized that there
existed a number of as yet unknown elements with atomic weights between the weights of already known
elements. By leaving vacancies for those elements he believed were undiscovered, he hit upon an
organizational scheme that seemed to vertically group elements with similar
properties. Among
elements with low atomic weights, he found that similar chemical characteristics recurred every seven
elements. Among heavier elements, he found that characteristics resurfaced every seventeen elements.
This phenomenon in which physical and chemical characteristics of elements are periodic functions of
their atomic weight is called the periodic law (and gives the periodic table
its name). In 1879,
Mendeleyev's periodic table received a powerful boost in general acceptance when it predicted the
existence of the elements gallium, germanium, and scandium.

Through time, Mendeleyev's periodic table has undergone some small changes. Many, many new elements have
been added. The discovery of the inert gases raised the number of elements between similar elements to
eight for the lighter elements and eighteen for the darker elements. In a few instances, scientists
have discovered that organization along atomic weights does not coincide with vertical similarities. In
such instances, as in the case of tellurium (Te) and iodine (I), similarity wins out over atomic weight
in determining organization.